While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No.3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at a much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency. The light-emitting layer is typically composed of at least one host material which does not significantly contribute to light emission and an emissive material (typically referred to as a dopant) which emits the desired color of light. It is also well known to use mixtures of compounds as co-hosts in LELs to control the characteristics and performance of the LEL.
Many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence. The excited singlet state is created when excitons formed in an OLED device transfer their energy to the excited state of the dopant. However, it is generally believed that only 25% of the excitons created in an EL device are singlet excitons. The remaining excitons are triplet, which cannot transfer their energy to the singlet excited state of a dopant. This results in a large loss in efficiency since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their energy to a dopant if it has a triplet excited state that is low enough in energy. If the triplet state of the dopant is emissive it can produce light by phosphorescence. In many cases singlet excitons can also transfer their energy to lowest singlet excited state of the same dopant. The singlet excited state can often relax, by an intersystem crossing process, to the emissive triplet excited state. Thus, it is possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission.
EP 1311141A1 discloses red light-emitting OLEDs with a mixture of a tertiary aromatic amine and a metal oxinoid compound as co-hosts with a red light emitting phosphorescent dopant. The metal oxinoid compound may be a gallium oxinoid. A related application, US 2003/0104243A1, discloses the same combination of co-hosts with a green fluorescent dopant.
US 2006/0063027A1 discloses OLED devices with a light-emitting layer containing a mixture of a hole-conducting material, including tertiary aromatic amines, and a metal oxinoid compound, including a gallium oxinoid, as co-hosts with a light-emitting dopant, including a phosphorescent dopant.
Commonly assigned US 2007/0134514A1 discloses OLED devices with a light-emitting layer containing a mixture of a tertiary aromatic amine and gallium complexes with 2-(2-hydroxyphenyl)pyridine ligands as co-hosts with a phosphorescent dopant.
Commonly assigned US 2007/0207345A1, US 2007/017342A1, US 20070166566A1 and US 2007/0003786A1 all disclose the use of gallium complexes with nitrogen bidentate ligands in layers other than light-emitting layers. Other references that disclose the use of metal, including gallium, complexes with bidentate nitrogen ligands include U.S. Pat. No. 6,420,057, JP2001081453, JP09003447 and JP2002110357.
Notwithstanding these developments, there remains a need for materials that will function as a host for a phosphorescent emitter in electroluminescent devices having improved luminous efficiency, low drive voltage and, most importantly, long operational lifetime.